Methods and apparatus for wire bonding with wire length adjustment in an integrated circuit

ABSTRACT

An integrated circuit is wire bonded in a manner such that there is consistent RF performance from integrated circuit package to integrated circuit package. Bond distances within the integrated circuit are measured, each corresponding to a wire bond to be formed. An area under a hypothetical wire bond profile is calculated as a function of the bond distances, a baseline wire length, and a baseline loop height. A wire is bonded across a given one of the bond distances to form a given one of the wire bonds. A wire bond profile for the given wire bond is provided having an area thereunder that is substantially equal to the calculated area.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/787,010, filed Feb. 25, 2004 now U.S. Pat. No. 7,086,148, thedisclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to the field of integratedcircuits and, more particularly, to wire-bonding operations performed onan integrated circuit during packaging.

BACKGROUND OF THE INVENTION

A radio frequency (RF) integrated circuit may include multipletransistor dies. A die attach machine is used to place the transistordies in an integrated circuit package. The die attach machine attemptsto place each die in its appropriate position within the package.However, every die attach machine has a specified tolerance allowingsome variance in the placement of the dies. Thus, the actual position ofa given die within the package may differ from its ideal position by anamount less than or substantially equal to the tolerance. A roboticbonding tool may then be used to wire-bond the dies to other circuitelements within the package, and to leads of a package leadframe. Such atool generally includes a surface/wire-feed detection system thatdetects bond pads or other bonding sites of a given die, and determinesthe height coordinates of these bond pads. The other circuit elements inan RF integrated circuit may include, for example, tuning capacitors.

A wire bond profile may be characterized as a side or profile view of awire extending from a first bond site to a second bond site. In an RFintegrated circuit, the wire bonds may carry high frequency signals.Certain types of RF integrated circuits, such as RF power transistors,are tuned through these wire bond profiles.

Since the placement of a transistor die within the package can vary fromone attach series to the next due to the die attach machine tolerance,an associated variation in the RF performance of the circuit may result.The variation in RF performance may be caused by unequal areas underwire bond profiles connecting similar elements of the integratedcircuit. In conventional practice, the wire bond length is generallyheld constant regardless of the bond distance between a given pair offirst and second bond sites. Therefore, as the bond distance changes,the area under the wire bond profile changes, causing inconsistent RFperformance from package to package.

Attempts to compensate for the tolerance of the die attach machineinclude increasing the size of the bond pads so that while wire lengthremains constant and the distance varies between bond sites, the wiremay still be bonded on the bond pad without greatly affecting the loopheight and the resulting area under the wire bond profile. Bondoperations may be performed on a portion of the bond pad closer to theperimeter of the die or on a portion of the bond pad closer to theinterior of the die. However, many die attach machine tolerances are toogreat to be fully compensated for by the size of the bond pad. Further,larger bond pads take up valuable room on the surface of the die thatmay be used for circuitry. Finally, die attach machines with very lowtolerances greatly increase the cost of packaging.

Traditional bonding tools have also been equipped with mechanicalfeatures called “close at loop” and “close at bond.” Close at bond isthe standard clamp close position in which the clamps will close afterthe second bond contact. Close at loop will close the wire clamps at thepeak position of the looping trajectory, reducing the variation fromwire to wire. However, such features are purely mechanical andassociated with a wire clamp mechanism. Since no actual calculation ofthe area under the wire profile is made using the bond distance, theprocess is not very accurate or repeatable.

Thus, a need exits for improved wire-bonding techniques, particularly inRF integrated circuit applications.

SUMMARY OF THE INVENTION

The present invention in an illustrative embodiment provides techniquesfor calculating and adjusting the area under a wire bond profile duringthe wire-bonding process, thereby allowing for consistent wire bondimpedance and RF performance from package to package for a given type ofintegrated circuit.

In accordance with one aspect of the invention, a method is provided forperforming a wire-bonding operation in an integrated circuit. Bonddistances within the integrated circuit are measured, each correspondingto a wire bond to be formed. An area under a hypothetical wire bondprofile is calculated as a function of the bond distances, a baselinewire length, and a baseline loop height. A wire is bonded across a givenone of the bond distances to form a given one of the wire bonds. A wirebond profile for the given wire bond is provided having an areathereunder that is substantially equal to the calculated area.

Advantageously, providing substantially equal areas under wire bondprofiles results in improved consistency in RF performance from packageto package for a given type of integrated circuit. Tolerances for thedie attach process also may be relaxed, thereby allowing for the use ofa less expensive die attach machine. Other advantages include thepotential for decreased manufacturing cycle time, and an ability toreduce bonding pad size. In addition, a software algorithmimplementation of the present invention may be configured bymodification of otherwise conventional bonding tool software, through acost-effective software upgrade. The results of the algorithm may beeasily verified and tracked using process control.

These and other objects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionof the illustrative embodiments thereof, which is to be read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a top cut-away view of a packagedintegrated circuit having wire bonds between dies, capacitors andpackage leads, according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating a magnified view of a die of theintegrated circuit of FIG. 1, according to an embodiment of the presentinvention;

FIG. 3 is a diagram illustrating wire bond profiles of the integratedcircuit of FIG. 1, according to an embodiment of the present invention;

FIG. 4 is a wire bond profile perspective view of an integrated circuit,according to an embodiment of the present invention;

FIG. 5 is a flow diagram illustrating a wire-bonding technique havingwire length adjustments, according to an embodiment of the presentinvention; and

FIG. 6 is a block diagram illustrating an example bonding systemsuitable for implementing a wire-bonding technique according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As will be illustrated in detail below, the present invention in theillustrative embodiment provides techniques for calculating andadjusting the area under a wire-bond profile during a wire-bondingprocess so that there is consistent RF performance from package topackage for a given type of integrated circuit.

Referring initially to FIG. 1, dies Q1, Q2, Q3, Q4, are disposed in apackaged RF integrated circuit 100 on a substrate 107. Integratedcircuit 100 is shown with an upper portion of the package removed sothat the internal elements and wires are visible. FIG. 1 shows die Q1disposed between capacitors C1, C2; die Q2 between capacitors C3, C4;die Q3 between capacitors C5, C6; and die Q4 between capacitors C7, C8.In this embodiment, dies Q1-Q4 are transistor dies and capacitors C1-C8are tuning capacitors of packaged RF integrated circuit 100. Dies Q1-Q4and tuning capacitors C1-C8 are disposed within an integrated circuitpackage. The package comprises a leadframe having leads illustrated byelements 110-1, 110-2, 110-3, 110-4.

A die attach machine places dies Q1-Q4 between tuning capacitors C1-C8as described above. The die attach machine attempts to place each die ina similar position relative to its respective adjacent tuning capacitorsand integrated circuit package leads. If each die is placed in a similarposition, the bond distances, or distances between dies and tuningcapacitors, and dies and package leads, would remain constant throughoutthe integrated circuit. However, due to die attach machine tolerances,the bond distances will vary from die to die, package to package, andeven wire bond to wire bond if one or more of the dies are skewed.

As shown in the figure, first set of wires 102-1 connects lead 110-1 tofirst tuning capacitor C1. Similarly, a second set of wires 104-1connects first tuning capacitor C1 to die Q1, a third set of wires 106-1connects die Q1 to second tuning capacitor C2, and a fourth set of wires108-1 connects die Q1 to lead 110-2. These wire sets are repeated foreach capacitor-die-capacitor arrangement. Wire sets 102-2, 104-2, 106-2,108-2 provide connections for die Q2 and its associated capacitors C3and C4. Wire sets 102-3, 104-3, 106-3, 108-3 provide connections for dieQ3 and its associated capacitors C5 and C6. Wire sets 102-4, 104-4,106-4, 108-4 provide connections for die Q4 and its associatedcapacitors C7 and C8.

Referring now to FIG. 2, a more detailed view of a portion of die Q1 isshown, illustrating the set of wires 104-1 extending out from the leftside of die Q1, and sets of wires 106-1 and 108-1 extending out from theright side of die Q1. In this embodiment, sets of wires 104-1 and 106-1connect to tuning capacitors, while set of wires 108-1 connects to theintegrated circuit package lead 110-2. Wires of the set 104-1 areindividually bonded to die Q1 at individual bond pads 112. Sets of wires106-1 and 108-1 are bonded to die Q1 at a bond strip 114. Bond pads,bond strips or other types of bonding sites can be utilized for each ofthe bonding areas on the dies, capacitors or leads.

FIG. 3 illustrates example wire bond profiles of the embodiment of thepresent invention shown in FIGS. 1 and 2. A first wire, from wire set102-1, connects lead 110-1 to tuning capacitor C1. A second wire, fromwire set 104-1, connects tuning capacitor C1 to die Q1. A third wire,from wire set 106-1, connects die Q1 to tuning capacitor C2. A fourthwire, from wire set 108-1, connects die Q1 to lead 110-2. An example ofwire bond loop height 113 is shown relating to the wire from wire set104-1. The wire bond loop height 113 is the magnitude of deviation, ormaximum perpendicular distance, of a wire from the first bond site.Thus, each wire has an associated bond loop height.

An example of bond distance 115 is also shown relating to the wire fromwire set 104-1. Bond distance is generally defined herein as thestraight line distance between first and second bond sites associatedwith a given wire bond. This distance may change depending upon factorssuch as die placement variation. Thus, each wire has an associated bonddistance. Further, an example of an area 116 under a wire bond profileis also shown in relation to the wire from wire set 104-1. This area isbounded by the wire, substrate 107, and portions of the dies and/orcapacitors disposed beneath the wire.

The following chart illustrates sample bond distance measurements andwire bond loop heights for wires in wire sets 102-1, 104-1, 106-1 and108-1, as determined for an exemplary implementation of this embodiment.All distance measurements and loop heights are in mils.

102-1 104-1 106-1 108-1 Wire Distance Height Distance Height DistanceHeight Distance Height 1 79.15 28.58 59.95 43.96 56.35 42.90 115.5527.98 2 79.10 28.56 59.75 44.26 56.45 42.78 115.40 28.18 3 78.90 28.2260.05 44.24 56.35 43.12 115.50 28.24 4 78.90 28.18 59.90 44.38 56.3542.96 115.45 28.18 5 79.10 27.54 59.80 44.16 56.30 42.84 115.50 28.30 679.20 27.86 59.70 44.08 56.45 42.66 115.35 28.50 7 78.65 27.90 60.3544.28 56.35 43.24 115.35 28.60 8 78.50 27.80 60.20 44.32 56.45 42.84115.50 28.32 9 78.95 28.44 59.95 44.28 56.35 43.02 115.45 29.06 10 79.0528.42 59.85 44.12 56.35 42.92 115.45 28.72 11 79.40 28.30 59.45 44.0856.40 43.12 115.50 28.86 12 79.50 27.98 59.35 43.82 56.50 42.82 115.3528.54 13 79.00 28.76 59.80 44.20 56.30 43.26 115.50 28.64 14 79.05 28.7459.75 44.36 56.40 42.90 115.45 28.56 15 78.80 28.48 59.80 44.36 56.4042.72 115.45 28.48 16 78.90 28.56 60.00 44.16 56.40 42.70 115.40 28.6417 79.45 27.92 59.40 44.24 56.35 43.10 115.50 28.48 18 79.55 27.80 59.3544.00 56.45 42.76 115.40 28.22 19 79.00 27.74 59.80 44.18 56.25 43.22115.35 28.30 20 79.35 27.82 59.50 44.40 56.45 42.78 115.45 28.54 Min78.50 27.54 59.35 43.82 56.25 42.66 115.35 27.98 Max 79.55 28.76 60.3544.40 56.50 43.26 115.55 29.06 Ave 79.08 28.18 59.79 44.19 56.38 42.93115.44 28.47

It is to be appreciated that these sample distance measurements and loopheights are presented by way of illustrative example only, and shouldnot be construed as limiting the scope of the invention in any way.

FIG. 4 shows a wire bond profile perspective view of an integratedcircuit configured in the manner described in conjunction with FIGS. 1,2 and 3. The figure illustrates that the wire bonds connecting similarintegrated circuit elements have substantially equal areas beneath theirwire bond profiles for consistent RF performance. For example, all wirebond profile areas beneath of second set of wires 104-1′, joining firsttuning capacitor C1′ and die Q1′, are substantially equal. Similarly,all wire bond profile areas beneath third set of wires 106-1′, joiningdie Q1′ and second tuning capacitor C2′, are also substantially equal,as are the areas under first set of wires 102-1′ and fourth set of wires108-1′. Through the techniques of the present invention, the area undereach wire bond profile remains substantially the same for similarconnections throughout the bonding process.

Referring now to FIG. 5, a flow diagram illustrates a wire-bondingtechnique utilizable to form the wire bond sets illustrated in FIGS. 1to 4. In step 502, the first bond site is aligned in the bonding tooland selected bond distances are measured. Each bond distance correspondsto a wire bond to be formed. The bond distance in the illustrativeembodiment may be viewed, for example, as the straight line distancebetween a first bond site and second bond site, although other types ofbond distances may be used. These bond sites may be disposed, forexample, on dies, capacitors, or on leads of the integrated circuitpackage. This technique may be implemented in the form of a softwarealgorithm compatible with otherwise conventional bonding tool software.For example, upon the loading of a program by an operator, such bondingtool software may align a first package and stops the bonding tool atthe first bond site in that package. The operator may be given an optionto measure a particular number of bond distances, such as up to 50 bonddistances.

In step 504, an average bond distance is calculated from the bonddistances measured in step 502. In step 506, an area under ahypothetical wire bond profile is calculated from the average bonddistance, using a wire length and loop height, both of which may havebeen previously programmed into the bonding tool, selected by theoperator, or otherwise determined. This wire length is an example ofwhat is more generally referred to herein as a “baseline” wire length.This loop height is an example of what is more generally referred toherein as a “baseline” loop height. The area may be calculated using anintegration technique, or other suitable area calculation technique, aswill be readily appreciated by those skilled in the art. The calculatedarea under the wire bond profile for the average bond distance is alsoreferred to herein as the “target area.” In computing the target area,functions of the bond distances other than averages may also be used.

In step 508, the wire-bonding process begins. In step 510, one of themeasured bond distances, the baseline wire length and the baseline loopheight are used to calculate an area under a hypothetical wire bondprofile for a given one of the wire bonds to be formed. In step 512,this area is compared to the target area. In step 514 the wire length,and thus the loop height, of the wire bond is adjusted during thewire-bonding process, so that the area under the wire bond profile forthe given wire bond is substantially equal to the target area. Thisadjustment may take place if the calculated area differs from the targetarea by more than a designated amount. Otherwise the given wire bond isformed using the baseline wire length. In step 516, steps 510, 512, and514 are repeated for each of the remaining wire bonds to be formed. Asnoted above, each of these wire bonds correspond to one of the bonddistances measured in step 502.

In the adjustment step 514, loop height may be modified by a loopcoefficient to accommodate varying wire lengths. The loop coefficientmay be, for example, a percentage applied to the length of the wire, asfollows:(Loop Coefficient (%)/100)*Wire LengthThe resulting number in mils may be added directly to the baseline loopheight value. Therefore, as the wire length gets longer the loop heightmotion will increase to pay out more wire and stabilize heights over thevarious lengths.

An implementation of the present invention as a software algorithm mayalso provide specific screen choices, based on the otherwiseconventional bonding tool software that the algorithm is incorporatedinto. Many different integration techniques may be utilized in themeasurement of the area under the wire bond profiles. Also, measurementsfor the determination of an average distance between bond sites may betaken before the bonding process begins for a series of packages. Thiswould result in wire bond profile areas that are substantially constantthrough each of the packages made. However, measurements may also betaken before the bonding process begins for each package. This may bedesirable if, for example, the only concern is the skew of the diewithin the package and it is not necessary to have constant wire bondprofile area from package to package.

Referring now to FIG. 6, a block diagram illustrates an example ofbonding system 600 in which a wire-bonding technique of the inventionmay be implemented. As illustrated, the system 600 comprises a bondingtool 602 coupled to a computer 604 which may comprise a processor 606and a memory 608. The bond distance and wire bond profile areacomputations of the invention may be performed at least in partutilizing software executed by processor 606 and stored in memory 608.Results of these computations are used in the bonding operations ofbonding tool 602.

Accordingly, as described herein, the present invention in theillustrative embodiment provides a wire-bonding technique that createssubstantially equal areas under wire bond profiles for similarconnections, thereby providing consistent RF performance.

The techniques are cost-effective, and the results are easily verifiedand tracked using process control. Also, tolerances for the die attachprocess may be relaxed, thereby permitting use of a less expensive dieattach machine and decreasing manufacturing cycle time.

Additional embodiments of the present invention may incorporate variousnumbers and combinations of transistor dies, tuning capacitors, packageleads, or other circuit elements, arranged in various configurationswithin a given integrated circuit. The positioning and number oftransistor dies, tuning capacitors and other elements will of courseresult in various numbers and configurations of wire bonds andassociated bonding sites. The techniques of the present invention mayalso be used in non-RF integrated circuits.

Therefore, although illustrative embodiments of the present inventionhave been described herein with reference to the accompanying drawings,it is to be understood that the invention is not limited to thoseprecise embodiments, and that various other changes and modification maybe made by one skilled in the art without departing from the scope orspirit of the invention.

1. An integrated circuit comprising: an integrated circuit package; aplurality of circuit elements disposed within the integrated circuitpackage; and a plurality of wire bonds; whereby the plurality of wirebonds are formed by the steps of: (i) measuring a plurality of bonddistances within the integrated circuit, each bond distancecorresponding to one of the plurality of wire bonds to be formed; (ii)calculating an area under a hypothetical wire bond profile as a functionof the plurality of bond distances, a baseline wire length, and abaseline loop height; and (iii) bonding a wire across a given one of theplurality of bond distances to form a given one of the plurality of wirebonds; the bonding step being configured to provide a wire bond profilefor the given wire bond having an area thereunder that is substantiallyequal to the calculated area in step (ii).
 2. The integrated circuit ofclaim 1, wherein the integrated circuit comprises a radio frequency (RF)integrated circuit.
 3. The integrated circuit of claim 1, wherein theintegrated circuit comprises a plurality of dies.
 4. The integratedcircuit of claim 3, wherein the integrated circuit comprises a pluralityof capacitors, one or more of the dies being disposed between acorresponding pair of the capacitors.